Fuel cells are considered a forward-looking, low-emission alternative to conventional energy generation methods. The polymer electrolyte membrane fuel cell (PEM) is of particular interest for mobile applications. A proton-conductive polymer membrane is the central component in this type of fuel cell. Nafion®, which is a perfluorinated polymer with sulfonic-acid side groups made by DuPont, and similar products from Asahi continue to be the dominant membrane materials for this purpose.
There are numerous examinations of the use of other polymers as membrane materials in fuel cells. However, these polymers are almost exclusively sulfonated materials, the proton conductivity of which can be attributed to sulfonic-acid groups.
The synthesis of perfluorinated polymers with phosphonic-acid side groups, based on perfluorovinyloxy-substituted phosphonic acids as monomers, which are then copolymerized with tetrafluoroethylene and perfluoropropylvinylether, are described in the publication (M. Yamabe, K. Akiyama, Y. Akatsuka, M. Kato. Novel phosphonated perfluorocarbon polymers. Eur. Polym. J. 36 (2000) 1035-41). The use of these types of polymers in fuel cells is described in U.S. Pat. No. 6,087,032.
Other fuel cell membranes from polymers with phosphonic-acid side groups are described for example in U.S. Pat. No. 5,422,411. Other polymers for use in fuel cells are described in U.S. Pat. No. 5,679,482. These are fluorine-free styrene copolymers with a mixed functionalization of sulfonic-acid and phosphonic-acid groups.
Polymer electrolyte membrane fuel cells (PEM fuel cells) consist of two electrodes, which are separated from each other by a proton-conducting membrane (polymer electrolyte membrane or proton exchange membrane). The electrodes consist e.g. of carbon mats, which are steamed with platinum and connected with each other via an exterior electric circuit. So that a conversion from hydrogen and oxygen to water can take place, the proton-conducting membrane must be moistened. Hydrogen fuel is constantly supplied to the anode. The cathode is constantly supplied with oxygen. Two types of PEM fuel cells are in development: low-temperature cells (up to approx. 90° C.) and high-temperature cells (up to approx. 180° C.).
The low-temperature cell was developed in the 60 s. Back then, a sulfonated polystyrene membrane served as the electrolyte. Since 1969, the Nafion® membrane developed by DuPont has been installed in the PEM fuel cells. Low-temperature cells react sensitively to carbon monoxide (CO). This gas can block the anode catalyst, which leads to a loss of power. The membrane must be moistened so that protons can be conducted.
High-temperature cells react insensitively to CO and other impurities. Higher working temperatures in the cell are interesting for the house energy since they enable a more effective use of the generated heat. Since the membrane conducts protons without water, it does not need to be moistened.
There are already suggested solutions with respect to the membranes for high-temperature cells. For example, U.S. Pat. No. 6,387,230 B1 suggests for high-temperature cells a series of membranes, which consist of a combination of polymers and organic or inorganic materials (inorganic-organic composite membranes).